Typically, gravity-feed intravenous administration apparatus includes a container for the liquid to be administered, a tube or conduit connected to the container for conducting the liquid to a hollow needle, and a control device connected in the conduit for controlling the rate of flow of liquid into the patient's vascular system. The control device generally takes the form of an adjustable clamp on the flexible conduit and includes a drip chamber to provide a visual indication of the rate at which the liquid is being administered. Such gravity-feed systems are useful for both infusion, wherein liquid such as glucose, salt solutions, etc., are administered and transfusions, wherein blood is administered. The present invention relates to both processes.
In the administration of intravenous fluids to a patient, controlling the flow rate of the liquid is of critical importance. A patient can suffer serious consequences should the amount of liquid received in a particular period of time exceed a safe limit. The flow rate depends, among other things, upon the difference in height between the top of the liquid in the bottle or flexible bag and the insertion point of the canula usually positioned in the patient's arm. Increasing that distance provides a greater hydrostatic pressure and increases the flow rate. The height of the top of the liquid above the patient varies as the contents of the liquid container decrases. This change in the height of the liquid results in a change of the flow rate. The present invention is concerned with minimizing the flow rate variation caused by changes in the amount of liquid in the container.
A prior art approach to this problem is found in U.S. Pat. No. 2,771,878 to Folland, et al. In this approach, instead of suspending the liquid container directly from a fixed support, the container is supported from a rigid vertically-extending rod which is secured to one end of a compression-type helically-wound cylindrical spring in a cylinder. The spring is calibrated so as to raise the container as a function of the cylinder and its liquid contents, whereby the top of the liquid in the container is maintained at a substantially constant height above the patient. While in theory this system operates as intended, it suffers from some practical disadvantages. Specifically, according to present standards, the bottom of an intravenous (I.V.) liquid container should be disposed approximately three feet above the point of infusion in a patient's arm. A bed-ridden patient's arm is typically disposed approximately two and one half feet above the floor. The commonly-used I.V. bottle has approximately six to ten inches of liquid therein. The total height from which a common gravity-feed system support hook must be located is therefore between six and six and one half feet. In the FOLLAND, et al. patent, the vertically compressable spring is interposed between the hook and the I.V. container. The spring is disposed in a cylinder which is approximately one foot long. In addition, the filled I.V. container, when supported from the spring, compresses the spring approximately nine inches. The Folland, et al. device thereby adds one and three quarter feet to the height of the hook which must therefore be disposed eight feet or more above the floor. This has proven to be too high to be reached by the average nurse whose height is less than five and one half feet.
It is also desirable to provide an alarm indication when the liquid contents of the I.V. container approach empty in order to avoid undesirable consequences from infusing air into the bloodstream. The present invention is concerned with providing a suitable alarm device of this type.